Fix Box V2 webhook signature validation

[ryankon]

This fixes #127.

Outside of the "update task assignment" assertion that requires the test user to confirm their email, I've managed to get all the specs to pass by doing the following:

1. Update webhook_validator_spec.rb to use the updated signature generation
2. Updated passwords in files_spec.rb and folders_spec.rb to comply with updated password complexity requirements

Also, the thumbnail test transiently fails, so I added a comment to it.

[ryankon]

@xhocquet Happy New Year! I wanted to follow up and see if we could get this merged in. Thank you!